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T O P I C R E V I E W

spaceuk

Mars Reconnaisance Orbiter (MRO) to launch August 10, 2005.

NASA's Next Leap in Mars Exploration Ready for Launch

NASA's Mars Reconnaissance Orbiter (MRO) is ready for a morning launch on Wednesday, Aug. 10. The MRO will arrive at Mars in March 2006 for a mission to understand the planet's water riddles and to advance the exploration of the mysterious red planet.

The mission's first launch opportunity window is 7:54 to 9:39 a.m. EDT, Wednesday. If the launch is postponed, additional launch windows open daily at different times each morning through August. For trips from Earth to Mars, the planets move into good position for only a short period every 26 months. The best launch position is when Earth is about to overtake Mars in their concentric racing lanes around the sun.

"The teams preparing this orbiter and its launch vehicle have done excellent work and kept to schedule. We have a big spacecraft loaded with advanced instruments for inspecting Mars in greater detail than any previous orbiter, and we have the first Atlas V launch vehicle to carry an interplanetary mission. A very potent and exciting combination," said NASA's Mars Exploration Program Director Doug McCuistion.

The mission lifts off from Launch Complex 41, Cape Canaveral Air Force Station, Fla. It is the first government launch of Lockheed Martin's Atlas V launch vehicle. "We're ready to fly, counting down through final procedures," said Chuck Dovale, director for expendable-launch-vehicle launches at NASA Kennedy Space Center (KSC), Fla.

When MRO arrives in March, it begins a half-year "aerobraking" process. The MRO will gradually adjust the shape of its orbit by using friction from carefully calculated dips into the top of the Martian atmosphere. MRO's primary science phase starts in November 2006.

"Mars Reconnaissance Orbiter will give us several times more data about Mars than all previous missions combined," said James Graf, project manager for the mission at NASA's Jet Propulsion Laboratory (JPL), Pasadena Calif.

Researchers will use the data to study the history and distribution of Martian water. Learning more about what has happened to the water will focus searches for possible past or present Martian life. Observations by MRO will also support future Mars missions by examining potential landing sites and providing a communications relay between the Martian surface and Earth.

The craft can transmit about 10 times as much data per minute as any previous Mars spacecraft. This will serve both to convey detailed observations of the Martian surface, subsurface and atmosphere by the instruments on the orbiter and enable data relay from other landers on the Martian surface to Earth. NASA plans to launch the Phoenix Mars Scout in 2007 to land on the far northern Martian surface. NASA is also developing an advanced rover, the Mars Science Laboratory, for launch in 2009.

The mission is managed by JPL, a division of the California Institute of Technology, Pasadena, Calif., for the NASA Science Mission Directorate. Lockheed Martin Space Systems, Denver, built the spacecraft and is the prime contractor for the project.

NASA's Launch Services Program at the KSC is responsible for government engineering oversight of the Atlas V, spacecraft/launch vehicle integration and launch day countdown management.

Ben

No earlier than the 11th now. Problems with a backup gyro on the Atlas 5 have prompted testing.

NASA Announces Mars Orbiter Launch Delay

The launch of NASA's Mars Reconnaissance Orbiter (MRO) has been postponed 24 hours. The new launch window is Thursday, August 11 from 7:50 to 9:35 a.m. EDT.

The launch was postponed due to a failure of a Redundant Rate Gyro Unit (RRGU) at the manufacturer. This unit is similar to two RRGU's that are part of the flight control system on the Atlas V launch vehicle at Launch Complex 41 at Kennedy Space Center, Fla.

The mission engineering team is evaluating whether the failure that occurred in the testing at the manufacturer has any effect on other RRGU's in the MRO's Atlas V.

For interested media, establishing remote cameras at Complex 41 will occur on Wednesday; leaving the KSC Press Site at 2 p.m. The imagery opportunity for the Atlas V rollout from the Vertical Integration Facility to the launch departs from the KSC Press Site at 10:15 p.m. EDT, tomorrow.

On launch day, Aug. 11, the KSC News Center will open at 4:30 a.m. EDT. Foreign national news media should meet at Gate 1 at Cape Canaveral Air Force Station at 5 a.m. to be escorted to the KSC Press Site. NASA TV live coverage of the launch begins at 5:30 a.m.

Robert Pearlman

NASA release

NASA's Multipurpose Mars Mission Successfully Launched

A seven-month flight to Mars began this morning for NASA's Mars Reconnaissance Orbiter. The mission will inspect the red planet in fine detail and assist future landers.

An Atlas V launch vehicle, 19 stories tall with the two-ton spacecraft on top, roared away from Launch Complex 41 at Cape Canaveral Air Force Station at 4:43 a.m. PDT. Its powerful first stage consumed about 200 tons of fuel and oxygen in just over four minutes, then dropped away to let the upper stage finish the job of putting the spacecraft on a path toward Mars. This was the first launch of an interplanetary mission on an Atlas V.

"We have a healthy spacecraft on its way to Mars and a lot of happy people who made this possible," said James Graf, project manager for Mars Reconnaissance Orbiter at NASA's Jet Propulsion Laboratory, Pasadena, Calif.

The spacecraft established radio contact with controllers 61 minutes after launch and within four minutes of separation from the upper stage. Initial contact came through an antenna at the Japan Aerospace Exploration Agency's Uchinoura Space Center in southern Japan.

Health and status information about the orbiter's subsystems were received through Uchinoura and the Goldstone, Calif., antenna station of NASA's Deep Space Network. By 14 minutes after separation, the craft's solar panels finished unfolding, enabling it to start recharging batteries and operate as a fully functional spacecraft.

The orbiter carries six scientific instruments for examining the surface, atmosphere and subsurface of Mars in unprecedented detail from low orbit. For example, its high-resolution camera will reveal surface features as small as a dishwasher. NASA expects to get several times more data about Mars from the orbiter than from all previous Martian missions combined.

Researchers will use the instruments to learn more about the history and distribution of Mars' water. That information will improve understanding of planetary climate change and will help guide the quest to answer whether Mars ever supported life. The orbiter will also evaluate potential landing sites for future missions. The Mars Reconnaissance Orbiter will use its high-data-rate communications system to relay information between Mars surface missions and Earth.

Mars is 72 million miles from Earth today, but the spacecraft will travel more than four times that distance on its outbound-arc trajectory to intercept the red planet on March 10, 2006. The cruise period will be busy with checkups, calibrations and trajectory adjustments.

On arrival day, the spacecraft will fire its engines and slow itself enough for Martian gravity to capture it into a very elongated orbit. The spacecraft will spend half a year gradually shrinking and shaping its orbit by "aerobraking," a technique using the friction of carefully calculated dips into the upper atmosphere to slow the vehicle. The mission's main science phase is scheduled to begin in November 2006.

The launch was originally scheduled for August 10, but was delayed first due to a gyroscope issue on a different Atlas V, and the next day because of a software glitch.

The mission is managed by JPL, a division of the California Institute of Technology, Pasadena, for the NASA Science Mission Directorate. Lockheed Martin Space Systems, Denver, prime contractor for the project, built both the spacecraft and the launch vehicle.

NASA's Launch Services Program at the Kennedy Space Center is responsible for government engineering oversight of the Atlas V, spacecraft/launch vehicle integration and launch day countdown management.

Robert Pearlman

NASA release

Robotic NASA Craft Begins Orbiting Mars for Most-Detailed Exam

With a crucially timed firing of its main engines today, NASA's new mission to Mars successfully put itself into orbit around the red planet.

The spacecraft, Mars Reconnaissance Orbiter, will provide more science data than all previous Mars missions combined.

Signals received from the spacecraft at 2:16 p.m. Pacific Time after it emerged from its first pass behind Mars set off cheers and applause in control rooms at NASA's Jet Propulsion Laboratory, Pasadena, Calif., and at Lockheed Martin Space Systems, Denver.

"This is a great milestone to have accomplished, but it's just one of many milestones before we can open the champagne," said Colleen Hartman, deputy associate administrator for NASA's Science Mission Directorate. "Once we are in the prime science orbit, the spacecraft will perform observations of the atmosphere, surface, and subsurface of Mars in unprecedented detail."

The spacecraft traveled about 500 million kilometers (310 million miles) to reach Mars after its launch from Florida on Aug. 12, 2005. It needed to use its main thrusters as it neared the planet in order to slow itself enough for Mars' gravity to capture it. The thruster firing began while the spacecraft was still in radio contact with Earth, but needed to end during a tense half hour of radio silence while the spacecraft flew behind Mars.

"Our spacecraft has finally become an orbiter," said JPL's Jim Graf, project manager for the mission. "The celebration feels great, but it will be very brief because before we start our main science phase, we still have six months of challenging work to adjust the orbit to the right size and shape."

For the next half-year, the mission will use hundreds of carefully calculated dips into Mars' atmosphere in a process called "aerobraking." This will shrink its orbit from the elongated ellipse it is now flying, to a nearly circular two-hour orbit. For the mission's principal science phase, scheduled to begin in November, the desired orbit is a nearly circular loop ranging from 320 kilometers (199 miles) to 255 kilometers (158 miles) in altitude, lower than any previous Mars orbiter. To go directly into such an orbit instead of using aerobraking, the mission would have needed to carry about 70 percent more fuel when it launched.

The instruments on Mars Reconnaissance Orbiter will examine the planet from this low-altitude orbit. A spectrometer will map water-related minerals in patches as small as a baseball infield. A radar instrument will probe for underground layers of rock and water. One telescopic camera will resolve features as small as a card table. Another will put the highest-resolution images into broader context. A color camera will monitor the entire planet daily for changes in weather. A radiometer will check each layer of the atmosphere for variations in temperature, water vapor and dust.

"The missions currently at Mars have each advanced what we know about the presence and history of water on Mars, and one of the main goals for Mars Reconnaissance Orbiter is to decipher when water was on the surface and where it is now," said JPL's Dr. Richard Zurek, project scientist for the mission. "Water is essential for life, so that will help focus future studies of whether Mars has ever supported life."

The orbiter can radio data to Earth at up to 10 times the rate of any previous Mars mission. Besides sending home the pictures and other information from its own investigations, it will relay data from surface missions, including NASA's Phoenix Mars Scout scheduled for launch in 2007 and Mars Science Laboratory in development for 2009.

DChudwin

NASA release

NASA Orbiter Finds Martian Rock Record With 10 Beats to the Bar

Climate cycles persisting for millions of years on ancient Mars left a record of rhythmic patterns in thick stacks of sedimentary rock layers, revealed in three-dimensional detail by a telescopic camera on NASA's Mars Reconnaissance Orbiter.

Researchers using the High Resolution Imaging Science Experiment camera report the first measurement of a periodic signal in the rocks of Mars. This pushes climate-cycle fingerprints much earlier in Mars' history than more recent rhythms seen in Martian ice layers. It also may rekindle debates about some patterns of rock layering on Earth.

Layers of similar thickness repeat dozens to hundreds of times in rocks exposed inside four craters in the Arabia Terra region of Mars. In one of the craters, Becquerel, bundles of a 10-layer pattern repeat at least 10 times, which could correspond to a known 10-to-one pattern of changes in the tilt of the planet's rotation axis.

"Each layer has weathered into a stair step in the topography where material that's more resistant to erosion lies on top of material that's less resistant to erosion," said Kevin Lewis of the California Institute of Technology, Pasadena, who is the lead author of a report on the periodic layering published in the Dec. 5 edition of the journal Science.

Some periodic change in the environment appears to have affected how resistant the rock-forming sediments became, perhaps from changes in what size of sand or silt particles were deposited by the wind, or from how the particles were cemented together after deposition.

Some of the individual layers are less than three feet thick.

The camera, called HiRISE for short, took pairs of images of each site from slightly different angles in orbit, providing the stereo information necessary for determining each layer's thickness.

"It's easy to be fooled without knowing the topography and measuring the layers in three dimensions," said Alfred McEwen of the University of Arizona, Tucson, principal investigator for the camera and a co-author of the new report. "With the stereo information, it is clear there's a repeating pattern to these layers."

Geologists commonly find "rhythms," or repeating patterns, in sedimentary layers on Earth. Determining the source of the rhythms can be difficult. Some result from annual or tidal cycles, or from episodic flooding that may not be periodic at all, but the role of longer-term astronomical cycles has been debated. One step in showing that astronomical cycles can leave their mark in sediments came from finding repeating five-layer sets in some terrestrial bedrock, matching a known five-to-one ratio of two cyclical variations in Earth's orbit.

Lewis and colleagues found something similar on Mars: "Our findings suggest that cycles of climate change led to the patterns we see recorded in the Mars rock layers today, possibly as a result of similar variations in Mars' orbit," he said. "Mars has a 10-to-one ratio in cycles of how its tilt changes -- smaller wobbles within larger packages. Sure enough, we see a 10-to-one ratio in one of these layered deposits. It's like trying to identify a song -- it's easier if there are multiple instruments playing different parts, rather than just a single rhythm."

In addition to having rhythm of 10 beats to the bar instead of Earth's five-beat pattern, Mars has characteristics that make it a good laboratory for studying how astronomical cycles affect climate. The tilt of Mars' axis varies much more than the axis of Earth, because Earth's relatively large moon provides a stabilizing effect. And, at least for most of its history, Mars has lacked the oceans and thick atmosphere that, on Earth, modulate the effects of orbital variations and add their own cyclical patterns.

The 10-beat pattern of Mars' wobble lasts about 1.2 million years. If the 10-layer bundles in Becquerel crater are indeed signatures of that cycle, the 10 or more bundles stacked on each other record about 12 million years when environmental conditions affecting sedimentation were generally steady except for effects of the changing tilt.

Robert Pearlman

NASA release

Mars Orbiter Completes Prime Mission

NASA's Mars Reconnaissance Orbiter has completed its primary, two-year science phase. The spacecraft has found signs of a complex Martian history of climate change that produced a diversity of past watery environments.

The orbiter has returned 73 terabits of science data, more than all earlier Mars missions combined. The spacecraft will build on this record as it continues to examine Mars in unprecedented detail during its next two-year phase of science operations.

Among the major findings during the primary science phase is the revelation that the action of water on and near the surface of Mars occurred for hundreds of millions of years. This activity was at least regional and possibly global in extent, though possibly intermittent. The spacecraft also observed that signatures of a variety of watery environments, some acidic, some alkaline, increase the possibility that there are places on Mars that could reveal evidence of past life, if it ever existed.

Since moving into position 186 miles above Mars' surface in October 2006, the orbiter also has conducted 10,000 targeted observation sequences of high-priority areas. It has imaged nearly 40 percent of the planet at a resolution that can reveal house-sized objects in detail, 1 percent in enough detail to see desk-sized features. This survey has covered almost 60 percent of Mars in mineral mapping bands at stadium-size resolution. The orbiter also assembled nearly 700 daily global weather maps, dozens of atmospheric temperature profiles, and hundreds of radar profiles of the subsurface and the interior of the polar caps.

"These observations are now at the level of detail necessary to test hypotheses about when and where water has changed Mars and where future missions will be most productive as they search for habitable regions on Mars," said Richard Zurek, Mars Reconnaissance Orbiter project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Included in the observations are hundreds of stereo pairs used to make detailed topography maps and classic images in support of other Mars missions. One image showed the Mars rover Opportunity poised on the rim of Victoria Crater and another of NASA's Phoenix Mars Lander during its descent to the surface. Orbiter data prompted the Phoenix team to change the spacecraft's landing site, and are being used to select the landing location for NASA's Mars Science Laboratory, which is scheduled for launch in 2011. For five months of Phoenix operations on Mars that ended in November, the Mars Reconnaissance Orbiter and NASA's Mars Odyssey orbiter shared the vital communications roles of relaying commands to the lander and data from Phoenix back to Earth.

The Mars Reconnaissance Orbiter has found repetitive layering in Mars' permanent polar ice caps. The patterns suggest climate change cycles continuing to the present. They may record possible effects of cyclical changes in Mars' tilt and orbit on global sunlight patterns. Recent climate cycles are indicated by radar detection of subsurface icy deposits outside the polar regions, closer to the equator, where near-surface ice is not permanently stable. Other results reveal details of ancient streambeds, atmospheric hazes and motions of water, along with the ever-changing weather on Mars.

Most observations from the orbiter will be discontinued for a few weeks while the sun is between Earth and Mars, which will disrupt communications. In December, the orbiter will begin a new phase, with science observations continuing as Mars makes another orbit around the sun, which takes approximately two Earth years.

"This spacecraft truly exemplifies the best in capabilities to support science and other Martian spacecraft activities," said Michael Meyer, lead scientist for the Mars Exploration Program at NASA Headquarters in Washington. "MRO has exceeded its own goals and our expectations. We look forward to more discoveries as we continue to look at the Red Planet in spectacular detail."

NASA's Jet Propulsion Laboratory in Pasadena manages the Mars Reconnaissance Orbiter for NASA's Science Mission Directorate, Washington. Lockheed Martin Space Systems, Denver, is the prime contractor for the project and built the spacecraft.

Robert Pearlman

NASA release

Scientists Find 'Missing' Mineral and Clues to Mars Mysteries

Researchers using a powerful instrument aboard NASA's Mars Reconnaissance Orbiter have found a long-sought-after mineral on the Martian surface and, with it, unexpected clues to the Red Planet's watery past.

Surveying intact bedrock layers with the Compact Reconnaissance Imaging Spectrometer for Mars, or CRISM, scientists found carbonate minerals, indicating that Mars had neutral to alkaline water when the minerals formed at these locations more than 3.6 billion years ago. Carbonates, which on Earth include limestone and chalk, dissolve quickly in acid. Therefore, their survival until today on Mars challenges suggestions that an exclusively acidic environment later dominated the planet. Instead, it indicates that different types of watery environments existed. The greater the variety of wet environments, the greater the chances one or more of them may have supported life.

"We're excited to have finally found carbonate minerals because they provide more detail about conditions during specific periods of Mars' history," said Scott Murchie, principal investigator for the instrument at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md.

The findings will appear in the Dec. 19 issue of Science magazine and were announced Thursday at a briefing at the American Geophysical Union's Fall Meeting in San Francisco.

Carbonate rocks are created when water and carbon dioxide interact with calcium, iron or magnesium in volcanic rocks. Carbon dioxide from the atmosphere becomes trapped within the rocks. If all of the carbon dioxide locked in Earth's carbonates were released, our atmosphere would be thicker than that of Venus. Some researchers believe that a thick, carbon dioxide-rich atmosphere kept ancient Mars warm and kept water liquid on its surface long enough to have carved the valley systems observed today.

"The carbonates that CRISM has observed are regional rather than global in nature, and therefore, are too limited to account for enough carbon dioxide to form a thick atmosphere," said Bethany Ehlmann, lead author of the article and a spectrometer team member from Brown University, Providence, R.I.

"Although we have not found the types of carbonate deposits which might have trapped an ancient atmosphere," Ehlmann said, "we have found evidence that not all of Mars experienced an intense, acidic weathering environment 3.5 billion years ago, as has been proposed. We've found at least one region that was potentially more hospitable to life."

The researchers report clearly defined carbonate exposures in bedrock layers surrounding the 1,489-kilometer-diameter (925-mile) Isidis impact basin, which formed more than 3.6 billion years ago. The best-exposed rocks occur along a trough system called Nili Fossae, which is 666 kilometers (414 miles) long, at the edge of the basin. The region has rocks enriched in olivine, a mineral that can react with water to form carbonate.

"This discovery of carbonates in an intact rock layer, in contact with clays, is an example of how joint observations by CRISM and the telescopic cameras on the Mars Reconnaissance Orbiter are revealing details of distinct environments on Mars," said Sue Smrekar, deputy project scientist for the orbiter at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

NASA's Phoenix Mars Lander discovered carbonates in soil samples. Researchers had previously found them in Martian meteorites that fell to Earth and in windblown Mars dust observed from orbit. However, the dust and soil could be mixtures from many areas, so the carbonates' origins have been unclear. The latest observations indicate carbonates may have formed over extended periods on early Mars. They also point to specific locations where future rovers and landers could search for possible evidence of past life.

The Applied Physics Laboratory led the effort to build the Compact Reconnaissance Imaging Spectrometer for Mars and operates the instrument in coordination with an international team of researchers from universities, government and the private sector. JPL, a division of the California Institute of Technology, Pasadena, manages the Mars Reconnaissance Orbiter mission for the NASA Science Mission Directorate in Washington. Lockheed Martin Space Systems, Denver, is the prime contractor for the project and built the spacecraft.

Robert Pearlman

NASA release

NASA Spacecraft Sees Ice on Mars Exposed by Meteor Impacts

NASA's Mars Reconnaissance Orbiter has revealed frozen water hiding just below the surface of mid-latitude Mars. The spacecraft's observations were obtained from orbit after meteorites excavated fresh craters on the Red Planet.

Scientists controlling instruments on the orbiter found bright ice exposed at five Martian sites with new craters that range in depth from approximately half a meter to 2.5 meters (1.5 feet to 8 feet). The craters did not exist in earlier images of the same sites. Some of the craters show a thin layer of bright ice atop darker underlying material. The bright patches darkened in the weeks following initial observations, as the freshly exposed ice vaporized into the thin Martian atmosphere. One of the new craters had a bright patch of material large enough for one of the orbiter's instruments to confirm it is water-ice.

The finds indicate water-ice occurs beneath Mars' surface halfway between the north pole and the equator, a lower latitude than expected in the Martian climate.

"This ice is a relic of a more humid climate from perhaps just several thousand years ago," said Shane Byrne of the University of Arizona, Tucson.

Byrne is a member of the team operating the orbiter's High Resolution Imaging Science Experiment, or HiRISE camera, which captured the unprecedented images. Byrne and 17 co-authors report the findings in the Sept. 25 edition of the journal Science.

"We now know we can use new impact sites as probes to look for ice in the shallow subsurface," said Megan Kennedy of Malin Space Science Systems in San Diego, a co-author of the paper and member of the team operating the orbiter's Context Camera.

During a typical week, the Context Camera returns more than 200 images of Mars that cover a total area greater than California. The camera team examines each image, sometimes finding dark spots that fresh, small craters make in terrain covered with dust. Checking earlier photos of the same areas can confirm a feature is new. The team has found more than 100 fresh impact sites, mostly closer to the equator than the ones that revealed ice.

An image from the camera on Aug. 10, 2008, showed apparent cratering that occurred after an image of the same ground was taken 67 days earlier. The opportunity to study such a fresh impact site prompted a look by the orbiter's higher resolution camera on Sept. 12, 2009, confirming a cluster of small craters.

"Something unusual jumped out," Byrne said. "We observed bright material at the bottoms of the craters with a very distinct color. It looked a lot like ice."

The bright material at that site did not cover enough area for a spectrometer instrument on the orbiter to determine its composition. However, a Sept. 18, 2008, image of a different mid-latitude site showed a crater that had not existed eight months earlier. This crater had a larger area of bright material.

"We were excited about it, so we did a quick-turnaround observation," said co-author Kim Seelos of Johns Hopkins University Applied Physics Laboratory in Laurel, Md. "Everyone thought it was water-ice, but it was important to get the spectrum for confirmation."

The ice exposed by fresh impacts suggests that NASA's Viking Lander 2, digging into mid-latitude Mars in 1976, might have struck ice if it had dug 10 centimeters (4 inches) deeper. The Viking 2 mission, which consisted of an orbiter and a lander, launched in September 1975 and became one of the first two space probes to land successfully on the Martian surface. The Viking 1 and 2 landers characterized the structure and composition of the atmosphere and surface. They also conducted on-the-spot biological tests for life on another planet.

Credit: NASA/JPL-Caltech/University of Arizona

The High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter took these images of a fresh, 6-meter-wide (20-foot-wide) crater on Mars on Oct. 18, 2008, (left) and on Jan. 14, 2009. Each image is 35 meters (115 feet) across. This crater's depth is estimated to be 1.33 meters (4.4 feet).

Images (not shown here) taken by the Thermal Emission Imaging System camera on NASA's Mars Odyssey orbiter and by the Context Camera on the Mars Reconnaissance Orbiter show that the impact that excavated this crater occurred sometime between Dec. 22, 2008 and July 5, 2008.

The impact exposed water ice from below the surface. It is the bright material visible in this pair of images. The change in appearance from the earlier image to the later one resulted from some of the ice sublimating away during the Martian northern-hemisphere summer, leaving behind dust that had been intermixed with the ice. The thickening layer of dust on top obscured the remaining ice. This crater is at 43.28 degrees north latitude, 164.22 degrees east longitude.

"NASA's Mars Exploration Program keeps bringing us closer to determining whether the Red Planet could harbor life in some form," NASA Administrator Charles Bolden said, "and it reaffirms Mars as an important future destination for human exploration."

Dark, finger-like features appear and extend down some Martian slopes during late spring through summer, fade in winter, and return during the next spring. Repeated observations have tracked the seasonal changes in these recurring features on several steep slopes in the middle latitudes of Mars' southern hemisphere.

"The best explanation for these observations so far is the flow of briny water," said Alfred McEwen of the University of Arizona, Tucson. McEwen is the principal investigator for the orbiter's High Resolution Imaging Science Experiment (HiRISE) and lead author of a report about the recurring flows published in Thursday's edition of the journal Science.

Some aspects of the observations still puzzle researchers, but flows of liquid brine fit the features' characteristics better than alternate hypotheses. Saltiness lowers the freezing temperature of water.

Sites with active flows get warm enough, even in the shallow subsurface, to sustain liquid water that is about as salty as Earth's oceans, while pure water would freeze at the observed temperatures.

"These dark lineations are different from other types of features on Martian slopes," said MRO project scientist Richard Zurek of NASA's Jet Propulsion Laboratory in Pasadena, Calif. "Repeated observations show they extend ever farther downhill with time during the warm season."

The features imaged are only about 0.5 to 5 yards or meters wide, with lengths up to hundreds of yards. The width is much narrower than previously reported gullies on Martian slopes. However, some of those locations display more than 1,000 individual flows. Also, while gullies are abundant on cold, pole-facing slopes, these dark flows are on warmer, equator-facing slopes.

The images show flows lengthen and darken on rocky equator-facing slopes from late spring to early fall. The seasonality, latitude distribution and brightness changes suggest a volatile material is involved, but there is no direct detection of one. The settings are too warm for carbon-dioxide frost and, at some sites, too cold for pure water. This suggests the action of brines which have lower freezing points. Salt deposits over much of Mars indicate brines were abundant in Mars' past. These recent observations suggest brines still may form near the surface today in limited times and places.

When researchers checked flow-marked slopes with the orbiter's Compact Reconnaissance Imaging Spectrometer for Mars (CRISM), no sign of water appeared. The features may quickly dry on the surface or could be shallow subsurface flows.

"The flows are not dark because of being wet," McEwen said. "They are dark for some other reason."

A flow initiated by briny water could rearrange grains or change surface roughness in a way that darkens the appearance. How the features brighten again when temperatures drop is harder to explain.

"It's a mystery now, but I think it's a solvable mystery with further observations and laboratory experiments," McEwen said.

These results are the closest scientists have come to finding evidence of liquid water on the planet's surface today. Frozen water, however has been detected near the surface in many middle to high-latitude regions. Fresh-looking gullies suggest slope movements in geologically recent times, perhaps aided by water. Purported droplets of brine also appeared on struts of the Phoenix Mars Lander. If further study of the recurring dark flows supports evidence of brines, these could be the first known Martian locations with liquid water.

MRO is managed by JPL for NASA's Science Mission Directorate in Washington. The University of Arizona's Lunar and Planetary Laboratory operates HiRISE. The camera was built by Ball Aerospace & Technologies Corp. in Boulder, Colo. Johns Hopkins University Applied Physics Laboratory in Laurel, Md., provided and operates CRISM.